Supported catalyst

ABSTRACT

The present invention relates to a Catalyst comprising a, preferably oxidic, core material, a shell of zinc oxide around said core material, and a catalytically active material in or on the shell, based on one or more of the metals cobalt, iron, ruthenium and/or nickel, preferably a Fischer-Tropsch catalyst, to the preparation of such a catalyst and the use thereof in GTL processes.

The invention relates to a heterogeneous catalyst, preferably aFischer-Tropsch (FT) catalyst suitable for GTL (gas-to-liquid)processes, comprising at least one catalytic metal on a support, to amethod for preparing such a catalyst and to processes using such acatalyst.

A catalyst containing cobalt oxide and zinc oxide for use in thesynthesis of C1-C3 aliphatic hydrocarbons is known from U.S. Pat. No.4,039,302.

U.S. Pat. No. 4,826,800 describes a process for preparing a catalystcomprising cobalt and zinc oxide for use after reductive activation as acatalyst in the conversion of synthesis gas to hydrocarbons. Thecatalyst is prepared by mixing a solution of a soluble zinc salt and asoluble cobalt salt with a precipitant such as ammonium hydroxide orammonium carbonate and recovering the precipitate.

U.S. Pat. No. 5,945,458 and U.S. Pat. No. 5,811,365 describe aFischer-Tropsch process in the presence of a catalyst composition of agroup VIII metal, e.g. cobalt, on a zinc oxide support. Such a catalystis made by first preparing the support by adding a solution of zinc saltand other constituents to an alkaline bicarbonate solution. Next, theprecipitate is separated from the bicarbonate solution by filtration toform a filter cake, which can thereafter be dried, calcined and loadedwith the group VIII metal. The catalyst material is then formed intotablets, which tablets are crushed to form particles with a size of250-500 μm, that can be used in a Fischer-Tropsch process. Additionalpost-treatments such as crushing, are required in order to obtain acatalyst powder for use in a slurry-phase process. However, the obtainedaverage particle size, as indicated above, is still relatively large.Moreover, a lack of strength results in crushing to irregularly shapedparticles and a broad particle size distribution. Catalysts with suchlarge irregularly shaped particles and a broad particle sizedistribution tend to be less suitable for processes involving a bubblecolumn, a slurry phase reactor or a loop reactor.

It has further been found that these conventional catalysts do notalways satisfy the requirements with respect to mass transfer and/orheat transfer, when used in a catalytic process.

In addition, it has been found that—when used in a slurry phaseprocess—separation properties, e.g. by filtration, are not particularlygood, since a broad particle size distribution results in a very densefiltercake.

Further it has been found that the dispersion behaviour of theseconventional catalysts—when used in a slurry phase process—is notparticularly good, since the catalyst particles tend to agglomerate.

Other problems with commercially available zinc oxide supports suitablefor loading with catalytic metal to form a catalyst, includeinappropriate particle size distribution (in particular with supportsobtained by precipitation), low surface area and pore volume, whichtypically makes them more difficult to impregnate, and severalimpregnation steps are required to deposit a reasonable amount of metalloading on the support. A low level of homogeneity of the metaldistribution is obtained, once the metal has been applied. Further, theintrinsic strength of commercial zinc oxide particles is relatively low,making them strongly subject to attrition when used in a slurry-phasereactor.

In WO-A 03090925 a catalyst has been described comprising a cobalt andzinc coprecipitate having a specific particle size distribution. Thecatalyst of said invention has a very good mass and heat transfer in GTLprocesses.

It is an object of the present invention to provide a novel catalyst,suitable for use in Fischer-Tropsch synthesis, that may be used as analternative to known catalysts, and which catalyst does not have atleast some of the disadvantages of the various prior art catalyst, suchas low strength and broad particle size distribution.

The invention is based on the surprising insight, that the use of acore-shell support based on an oxidic core and a zinc oxide shell,provides an excellent basis for preparing a zinc oxide based catalysthaving a better attrition resistance than previously known catalysts.

Accordingly, the present invention relates to a catalyst, more inparticular a Fischer-Tropsch catalyst, comprising a, preferably oxidic,core material, a shell of zinc oxide around said core material, and acatalytically active material in or on the shell, based on one or moreof the metals cobalt, iron, ruthenium and/or nickel.

It has been found that a catalyst according to the present invention hasvery favourable properties for use in catalytic processes, more inparticular gas to liquid FT-processes, wherein liquid hydrocarbons areprepared from synthesis gas. More in particular the catalyst of theinvention has a superior strength, resulting in a very low abrasion ofthe material, thereby making it very suitable for those catalyticreactors that require strong catalyst particles.

Further, the catalyst according to the invention has been found to haveparticularly good mass and/or heat transfer properties, when used in acatalytic process.

A catalyst according to the invention has been found to be particularlyfavourable for use in a stirred slurry-phase reactor, bubble-columnreactor, loop reactor or fluid-bed reactor.

A catalyst according to the invention shows very good flow properties indry form and/or when used in a stirred slurry reactor, and gooddispersibility properties with the reactants in the reaction mixture.The catalyst of the invention can be prepared in very appropriateparticle size distribution, as indicated by the free-flowing propertiesof the dried catalyst, as can be observed, for example, when thecatalyst is kept in a storage flask. This result is at least partlyobtained by the fact that the oxidic core can be prepared separatelybefore the application of the zinc oxide shell.

A catalyst according to the invention shows very favourable separationproperties and can for example very suitably be separated from thereaction mixture by filtration.

A catalyst according to the invention has an extremely good balancebetween activity and separation properties.

Preferably the catalyst of the present invention has mainly (i.e. atleast 75 vol. %) pores having a diameter in the range of 1-15 nm. Muchpreferred is a catalyst having essentially no pores with a diameter ofless than 5 nm (in particular less than 5% of the pore volume formed bypores with a diameter of less than 5 nm). It has been found that such acatalyst has particularly good diffusion properties for reactant andproduct. Such a catalyst has also been found to be highly selectivetowards the Fischer-Tropsch reaction.

Very good results have been achieved with a catalyst having a porevolume of less than 0.5 ml/g. The pore volume is preferably at least0.05 ml/g. Particularly suitable is a catalyst with an pore volume ofless than 0.45 ml/g.

The pore volume of the catalyst is determined by nitrogen adsorption(N₂-BET), measured on an Ankersmit Quantachrome Autosorb-6 apparatus,after degassing the sample at 180° C. to a pressure of 3.3 Pa (25mTorr).

Such a catalyst has been found to have particularly good physicalstrength properties, which is advantageous in applications in varioustypes of reactors, including slurry-phase reactors, loop-reactors,bubble-column reactors and fluid-bed reactors.

Also the surface area—as determined by nitrogen adsorption (N₂-BET) byan Ankersmit Quantachrome Autosorb-6 apparatus, after degassing at 180°C. down to a pressure of 3.3 Pa (25 mTorr), can be chosen within thewide range, depending upon the intended purpose. For a Fischer-Tropschprocess, this parameter may for example be chosen in the range of 1-500m²/g. Preferably a catalyst has a surface area in the range of 5-160m²/g. Very good results have been achieved with a catalyst having asurface area in the range of 5-150 m²/g.

A preferred catalyst according to the invention is a particulatematerial wherein the particles have a more or less spherical geometry.Such a catalyst has been found to have very good strength and separationproperties, and a relatively high attrition resistance during use.

The composition of the catalyst can be varied widely, which compositionthe skilled professional will know to determine, depending upon theintended purpose.

The catalyst may essentially consist of cobalt, iron, ruthenium and/ornickel as the metallic component. It is however also possible that thecatalyst contains one or more other components, such as components thatare commonly employed as promoters in Fischer-Tropsch catalysts. Thecatalyst may also contain one or more promoters, for example hafnium,platinum, zirconium, palladium, rhenium, cerium, lanthanum or acombination thereof. When present, such promoters are typically used inan atomic ratio of metallic component to promoter of up to 10:1.

The catalyst according to the invention contains a core that preferablycomprises oxidic materials, for example oxides based on silicon (Si),aluminium (Al), gallium (Ga), zirconium (Zr) and titanium (Ti), orcombinations thereof. In the preferred embodiment, aluminium isparticularly preferred.

In an other embodiment, the internal core comprises other materials, forexample carbides (e.g. silicon carbide) or clay-based structures (e.g.kaolins and montmorillonites).

In general the catalyst may be prepared by a method wherein a zinc oxidelayer is applied on the surface of the core material, optionally afterapplying an intermediate layer of another oxide, such as silica,tungsten oxide, or alumina. Between the application of the variouslayers, it is possible to wash and/or dry and/or calcine the material,however, this is not necessary.

After the core-shell support has been produced, the catalytically activematerial is applied thereon by suitable applications means, such asimpregnation, deposition precipitation, or by use of the so-calledlayer-by-layer method. In general, a salt of the cobalt, iron, rutheniumand/or nickel metal to be applied as catalytic material is brought ontothe zinc oxide surface by suitable means, followed by calcination andhydrogenation to produce the metal based catalyst.

Various methods, such as spray drying, are suitable for the applicationof the zinc oxide shell on the core. It is preferred to use a methodbased on the so-called Layer-By-Layer (LBL) method.

The present invention further relates to a method for preparing acatalyst as discussed above, by depositing a zinc oxide layer as a shellonto the core material particles, thereby applying electrostaticdeposition of at least one material onto another by utilizing chargereversal, including the use of ionic charge reversing agents to producea suitable catalyst support precursor by the use of the Layer-By-Layer(LBL) method.

Examples of this prior art LBL technique are described in Valtchev etal, Microporous and Mesoporous Materials, 43 (2001) 41-49; Wang et al.,Chemical Communications, 2161 (2000) and Millward et al., ChemicalCommunications, 1994 (2001). Also in Hoogeveen et al, Polyelectrolyteadsorption on Oxides I and II, J of Colloid and interface Science 182,133-145 (1996), and 182, 145-157 (1996) the adsorption of chargedpolyelectrolytes on oxide surfaces has been discussed.

U.S. Pat. No. 5,208,111 describes multi-layered layer elements appliedto supports using materials of opposite charge in consecutive layers. InU.S. Pat. No. 6,022,590 to Ferguson, the stepwise formation ofmultilayered structures is described, involving the alternate adsorptionof a cationic polyelectrolyte and anionic sheets of a silicate clay ontoa substrate.

In general, the use of the LBL method implies the alternate adsorptionof oppositely charged ionic species onto the surface of a substrate (thecore material), thereby reversing the charge thereof, as discussedhereafter.

Advantages of the LBL method in preparing the catalyst of the presentinvention include excellent control over layer thickness, the ability toincorporate layers of varying chemical composition, as well as the factthat the method is experimentally straightforward and can be performedat room temperature in an aqueous medium. The deposition reactions arefast even at room temperature.

After loading the core material with at least one charged ionic species,the coated core material is optionally provided with an oxide layer, byapplying a precursor for the oxide (such as silica, tungsten oxide, oralumina), prior to applying the zinc oxide, preferably as a colloidalsolution to the core material.

Materials for applying the intermediate oxide layer are suitably thegeneral oxide precursors, such as colloidal oxide solutions,polyoxometalcations or polyoxometallates.

Loading of charged ionic species and ZnO (with or without an oxide) istypically performed in a series of consecutive steps, followed bycalcination, as desired. In a subsequent step, the so-obtained supportmaterial is loaded with the catalyst precursor metal selected fromcobalt, iron, ruthenium and nickel, by incipient wetness impregnation,using an aqueous solution of a salt of the metallic component. In thisstage also the promoter may be applied.

This invention specifically refers to a preferred method of preparingthe catalyst by loading the core material with at least one chargereversing ionic species, dispersing the coated core material in amolecular or colloidal oxide precursor solution, such as of silica,treating the material again with at least one charge reversing ionicspecies and dispersing the said treated core material into a colloidalZnO solution.

This method has been found to be particularly suitable for preparing acatalyst as described above.

In a more specific embodiment, the invention concerns the preparation ofa robust support, namely alumina, with zinc oxide using layer-by-layercontrolled surface coating. It additionally discloses the deposition ofa second inorganic oxide between the alumina and zinc oxide layers as anintermediate between the zinc oxide layer and the substrate particle.These depositions rely on charge reversal, and the charge on thesubstrate particle can be altered as desired by using charged ionicmaterials such as poly diallyldimethylammonium chloride (denotedPDADMAC) or poly sodium styrene sulfonate (PSS) in aqueous solutions.

The charged ionic species, or charge reversing agents, that may be usedin the present invention include monomeric species, oligomeric materialsand low, medium, and high molecular weight polymers, for example in therange of up to about 1,000,000, more in particular of about 200 to about1,000,000. Monomeric species may be selected from various suitable ionicspecies, such as diallyldimethylammonium chloride or styrene sulfonicacid salt and the like. An example of a cationic inorganic oxideprecursor would be aluminum chlorohydrol (also known as the Keggin ion);an example of an anionic polyoxometallate is ammonium metatungstate.

Examples of polymeric species capable of forming large polyanions, whenionized, are well known. A preferred polymeric species is awater-soluble vinyl polymer, or an alkali metal or ammonium saltthereof, or an alkali metal or ammonium salt of polysilicic acid.Specific examples include poly (acrylic) acids, poly (methacrylic)acids, substituted poly (acrylic acid), substituted poly (methacrylicacid), or an alkali metal or an ammonium salt of any of these acids. Onecommercially available anionic species is sodium polyacrylate.

Further examples of suitable polymeric species useful in the presentinvention are disclosed in U.S. Pat. No. 5,006,574. One usefulwater-soluble cationic polymeric material is a diallyl quaternaryammonium polymer salt. This cationic polymer is characterized by a highdensity of positive charge. Preferably, the polymer does not havenegative groups such as carboxyl or carbonyl groups.

U.S. Pat. No. 5,006,574 also discloses other quaternary ammoniumcationic polymers obtained by copolymerizing an aliphatic secondaryamine with epichlorohydrin. Still other water-soluble cationicpolyelectrolytes are poly (quaternary ammonium) polyester salts thatcontain quaternary nitrogen in a polymeric backbone and are chainextended by the groups. They are prepared from water-soluble poly(quaternary ammonium salts) containing pendant hydroxyl groups andbi-functionally reactive chain extending agents. Such polyelectrolytesare prepared by treating N,N,N′,N′-tetraalkylhydroxyalkylene diamine andan organic dihalide such as dihydroalkane or dihaloether with an epoxyhaloalkane. Other water-soluble cationic polyelectrolytes arepolyamines, such as for instance polyallylamine hydrochloride, andalkylphosphonium salts.

Cationic polymeric species are also commercially available. For instancea cationic oligomer is marketed by Calgon Corp. under the trademark“CALGON 261” and another marketed by Nalco Chemical Co. under thetrademark “NALCO 7607”, and poly(sodium 4-styrene sulfonate) isavailable from National Starch and Chemical under the trademark “Flexan130”.

In the deposition phase, a solution of the ionic charge reversingspecies to be deposited is first prepared. The pH of this solution canbe adjusted as desired to control surface charge characteristics. Inaddition, an inorganic salt such as sodium chloride can be dissolved inthe said solution to control the ionic strength of the solution. Ameasured amount of substrate (core material) is added to the saidsolution, and the mixture stirred at room temperature for the desireddeposition time (typically 1-30 minutes). Following deposition, thesubstrate is collected from solution by filtration, and washed with anexcess of deionized water to remove excess, unattached charged ionicspecies.

The substrate is then re-slurried in a solution of the second coatinglayer that possesses a charge opposite to that of the first chargedspecies. The deposition process is repeated, and the substrate collectedin the same way. This series of deposition steps can be repeated as manytimes as desired by alternately subjecting the substrate to positivelyand negatively charged ionic species.

The preferrred material used to produce a positive surface charge on thesubstrate is poly-(diallyldimethylammonium chloride), denoted PDADMAC.In this material, the diallyldimethylammonium fragment confers thepositive charge on the substrate surface, and the negative counter ionis the chloride anion. PDADMAC is hence considered to be the “positivepolymer”. This material is available commercially with a variety ofmolecular weights, and may be used here with a molecular weight ofapproximately 200 to 1,000,000.

The preferred polymer used to provide a negative surface charge is poly(sodium 4-styrene sulfonate), denoted PSS. In this case the positivecounter ion is sodium, and the styrene sulfonate confers a negativesurface charge to the substrate. PSS may be used with a molecular weightof up to approximately 1,000,000, and can be used in the form of a solidor aqueous solution.

The inorganic materials used for this work are preferably colloidal zincoxide and colloidal silica, with a preferred particle size of <150 nm.Other inorganic oxides may be used with equal success.

The polymeric materials are used to manipulate the charge on thesubstrate particles to promote the deposition of the inorganic oxides.In order to make the oxide deposition more effective, the substrate ispreferably treated with PDADMAC.

It is to be noted, that it is possible to repeat the application of thevarious layers one or more times, with or without intermediatefiltration by sequential addition of controlled amounts of chargereversing agents. This gives the possibility to regulate the thicknessof the various layers, the diameter of the particles and the attritionbehavior.

When the deposition process has been completed, the sample is calcinedin air to remove the charged layers, leaving a material composedprimarily of inorganic oxides. Following calcination, the materials maybe recovered and examined by elemental analysis. Certain propertymeasurements may also be undertaken, such as the measurement of particlesize distribution after an attrition test. In addition, impregnation ofthe support with a suitable metal precursor, such as cobalt, iron,ruthenium and/or nickel, may be performed via standard techniques priorto performance evaluation in a selected catalytic reaction.

A typical experimental procedure is detailed below as Example 1 fordepositing a combination of silica and zinc oxide on alumina. Theisoelectric point of oxides and hydroxides of aluminum can varyconsiderably depending on composition, form and experimental conditions,mostly in the pH range 5 to 10 (Parks, Chemical Reviews (1965), pages177-198).

The present invention further relates to the use of a catalyst accordingto the invention in a slurry reactor, a loop reactor, a bubble-columnreactor or a fluid-bed reactor. The present invention further relates tothe use of a catalyst according to the invention in a Fischer-Tropschprocess or a functional group hydrogenation process, such as nitrilehydrogenation to amines.

The invention is further illustrated by the following examples.

EXAMPLE 1 Catalyst Preparation

A solution was prepared consisting of 1.5 g PSS (sodium polystyrenesulfonate, MW 70,000) in 114 g of 0.1 M aqueous NaCl, and the pHadjusted to ˜5 using 0.1 M aqueous hydrochloric acid. To this solutionwas added 30 g of Condea SB Alumina, and the mixture stirred for 15minutes at room temperature. After 15 minutes, the substrate wasrecovered by filtration and washed with an excess of deionized water.

The substrate was then treated with a solution comprising 3 g PDADMAC(poly diallyldimethylammonium chloride, MW 100,000 to 200,000) in 114 gof 0.1 M aqueous NaCl, adjusted to approximately pH 9 using 0.1 Mammonia solution. After stirring for 15 minutes at room temperature, thesubstrate was recovered by filtration and washed with an excess ofdeionized water.

The substrate was then slurried in 1% colloidal SiO₂ (Nalco 2327, 20 nmparticle size) in 0.1 M aqueous NaCl. The pH was not adjusted (pH ˜9).After stirring for 15 minutes, the solid was recovered by filtration andwashed with an excess of deionized water.

The substrate was then treated with a solution comprising 3 g PDADMAC(poly diallyldimethylammonium chloride) in 114 g of 0.1 M aqueous NaCl,adjusted to approximately pH 9 using 0.1 M ammonia solution. Afterstirring for 15 minutes at room temperature, the substrate was recoveredby filtration and washed with an excess of deionized water.

The substrate was then slurried in 1% colloidal ZnO (Nyacol DP5370, 50nm particle size) in 0.1 M aqueous NaCl. The pH was not adjusted. Afterstirring for 15 minutes, the solid was recovered by filtration andwashed with an excess of deionized water.

The substrate was then treated with 2 further treatments of(PDADMAC+ZnO) applied in an alternating manner as described above. Thecatalyst support was then dried at 90° C. in air. At this point, thecomposition of the material could be described as follows:Al₂O₃+PSS+PDADMAC+SiO₂+(PDADMAC+ZnO)₃

Following calcination in flowing air at 600° C., the catalyst wascharacterized for its chemical composition and physical properties. Thesilicon content was determined to be 1.5% w/w Si on a VF basis,corresponding to 3.2% w/W SiO₂. The zinc content was determined to be12.4% w/w Zn on a VF basis, corresponding to 15.4% w/w ZnO.

The so-obtained coated support was subsequently loaded with 20% cobalt,by porevolume impregnation, thereby dissolving cobaltnitrate in therequired volume of demiwater and impregnating this to the supportmaterial. After drying at 110° C., the material was calcined at 500° C.for 5 hours.

The analytical data of this catalyst are presented in table 1.

EXAMPLE 2 Catalyst Preparation

A zinc oxide-coated alumina material was prepared in an analogous mannerto Example 1, except that no silica was used. After calcination in air,the final zinc content was determined to be 2.0% w/w Zn, correspondingto 2.5% w/w ZnO.

Other chemical and physical properties of the ultimate catalyst arepresented in table 1.

EXAMPLE 3 Catalyst Preparation

A 1% w/w solution of PSS was prepared in 0.1 M aqeous NaCl, and the pHadjusted to ˜5 using 0.1 M aqueous hydrochloric acid. To 200 ml of thissolution, 30 g of Condea SB Alumina was added, and the mixture stirredfor 15 minutes at room temperature. After 15 minutes, the substrate wasrecovered by filtration and washed with an excess of deionized water.

The substrate was then slurried in 200 ml of a 1% w/w solution ofPDADMAC in 0.1 M aqeous NaCl, adjusted to pH 9.5 using 0.1 M ammoniasolution. Contact time was 15 minutes at room temperature undercontinuous agitation. Following this treatment, the substrate wascollected by filtration and washed with deionized water to removeexcess, unattached polymer.

The substrate was then slurried in 1% colloidal ZnO (Nyacol DP5370, 50nm particle size) in 0.1 M aqueous NaCl. The pH was not adjusted. Afterstirring for 15 minutes, the solid was recovered by filtration andwashed with an excess of deionized water.

The substrate was then treated with 2 further treatments of(PDADMAC+ZnO) applied in an alternating manner as described above. Thecatalyst support was then dried at 90° C. in air. At this point, thealumina composition can be described as Al₂O₃+PSS+(PDADMAC+ZnO)₃.Following calcination in flowing air at 550° C., the catalyst wascharacterized for its chemical composition. Elemental analysis showed13.0% w/w Zn on a volatile free basis, corresponding to 16.2% w/w ZnO.

EXAMPLE 4 Catalyst Preparation—Comparative Experiment

A metal solution (1000 ml) containing 21.0 g/l cobalt and 64.2 g/l zincwas prepared by dissolving 292.4 g of Zn(NO₃)₂.9H₂O and 103.8 g ofCo(NO₃)₂.6H₂O in 1000 ml of demineralised water. The base solution wasprepared by dissolving 142 g of (NH₄)₂CO₃ in 1000 ml of demineralisedwater. The metal and base solution were injected simultaneously at equalflow rates (1000 ml/hr) into a well stirred, baffled precipitationvessel containing 1750 ml of demineralised water. The temperature duringprecipitation was maintained at 75° C.

The pH was kept-constant at pH 6.2 by providing acid solution andalkaline solution at equal addition rates.

The resulting precipitate was washed with demineralised water and driedovernight at 110° C. The dried catalyst was heated from room temperaturewith 150° C./hr to 500° C. and calcined for 5 hours at 500° C.

The chemical and physical properties of this catalyst are presented intable 1. TABLE 1 Physical and chemical properties of the catalysts.Comparative Catalyst Catalyst catalyst Example 1 Example 2 (Example 4)Cobalt content wt % 19.3 20.0 20 Zn content wt % 12.4 2.0 80 Si contentwt % 1.5 — — BET-surface area m²/g 106 133 28 N₂ pore volume ml/g 0.290.34 0.19 Particle size distribution D(v.0.9)¹ μm 92 92 30.3 D(v.0.5)¹μm 38 38 23.1 D(v.0,1)¹ μm 8 8 17.9 Span¹ 2.3 2.3 0.5 Crystallite size²Å 140 137 150 Fines upon attrition³ % 2 n.d. 4 ad 1: The span iscalculated from the measured Malvern particle size distribution andgives an indication for the broadness of the particle size distribution,as is defined as follows:${Span} = \frac{{D\lbrack {v,0.9} \rbrack} - {D\lbrack {v,0.1} \rbrack}}{D\lbrack {v,0.5} \rbrack}$wherein:

-   D[v,0.9]=particle size (μm) below which 90% of particles exists (in    Malvern volume particle size distribution).-   D[v,0.5]=particle size (μm) below which 50% of particles exists (in    Malvern volume particle size distribution).-   D[v,0.1]=particle size (μm) below which 10% of particles exists (in    Malvern volume particle size distribution).

ad 2: The Co₃O₄ crystallite size, as reported in table 1, is calculatedfrom the XRD spectrum, particularly from the d=2.03 line in the XRDpattern (CuKα-radiation).

Ad 3: The physical strength of the powder particles was determined in aliquid-phase attrition test, by slurrying 5.0 grams of catalyst powderin 200 mls of demiwater, and treating the slurry in a blender (Waring,type 33BL79), operating at 15,000 RPM for 6 minutes. Fines were definedas the particles below 5 μm.

The cobalt content herein was measured by X-ray fluorescence.

EXAMPLE 5 Measurement of the Particle Size Distribution

The particle size distribution of a catalyst according to the inventionwas measured on a Malvern Mastersizer MS 20.

The sample vessel of the apparatus was filled with demineralized water,and diffraction of measuring-cell filled with water was determined (forbackground correction). An appropriate amount of catalyst powder wasthen added to the sample vessel, which was treated in ultrasonic bathfor 3 minutes (25% of max. output u.s. power) and stirring (50% of max.stirring speed), prior to the measurement. After this treatment, thesample was measured and the measured diffraction signal was correctedfor the ‘background’ measurement.

Calculation of particle size distribution was done using the followingparameters: Model: Model Independent; Presentation: 1907; Particle sizedistribution: Volume distribution.

EXAMPLE 6 Activity Test

A sample (2 ml) of the catalyst of Example 1 was diluted with 8 ml inertalumina and loaded into a fixed-bed reactor (9 mm diameter). Thecatalyst was first heated at 250° C. (60° C./h) under air. After a dwellfor 2 hours at 250° C., the air was replaced by nitrogen, applying thiscondition for 0.2 hours. The reduction was started by introducing carbonmonoxide (at 250° C.) for 3.5 hours. The carbon monoxide was thenreplaced by nitrogen, dwell for 0.2 hours. In a subsequent step, thereduction was completed under hydrogen for 1.5 hour, still at 250° C.The reactor was then cooled down to a temperature below 90° C. TheFischer-Tropsch test was started up by feeding syngas(hydrogen/carbonmoxide ratio 2:1) to the reactor at GHSV 8000 h⁻¹. Thereactor was then slowly heated up until the required CO conversion wasobtained.

After 40 hours on stream a C5+ productivity of 493 g/litre ofcatalyst/hr was obtained at a temperature of 238° C.

EXAMPLE 7 Activity Test—Comparative Test

A sample of catalyst (2 ml) made according to Example 4 (comparativepreparation) was reduced and activated according to the same method asdescribed in Example 6. After such activation, this catalyst showed aC5+ productivity of 558 g/litre of catalyst/hr, obtained at atemperature of 225° C.

1. Catalyst comprising a, preferably oxidic, core material, a shell ofzinc oxide around said core material, and a catalytically activematerial in or on the shell, based on one or more of the metals cobalt,iron, ruthenium and/or nickel.
 2. Catalyst according to claim 1, whereinthe said core material is selected from group silica, alumina, silicaalumina, titania, zirconia, Si-carbides, synthetic or natural claymaterials and combinations of two or more of these materials. 3.Catalyst according to claim 1, wherein the amount of catalyticallyactive material is between 5 and 50 wt. % of the total weight of thecatalyst.
 4. Catalyst according to claim 1, wherein the shell of zincoxide is between 1 and 30 wt. % of the combined weight of the core andthe shell.
 5. Catalyst according to claim 1, wherein the surface area ofthe active catalyst is less than 500 m²/g, preferably in the range of5-160 m²/g.
 6. Catalyst according to claim 1, predominantly comprisingparticles with a spherical geometry.
 7. Catalyst according to claim 1,wherein the catalytically active material is active in a Fischer-Tropschprocess.
 8. Method for preparing a catalyst comprising cobalt, iron,ruthenium and/or nickel on a zinc oxide core-shell support as defined inclaim 1, wherein a solid core material is provided with a shell of zincoxide, followed by application of one or more of the metals cobalt,iron, ruthenium and/or nickel precursor, and converting this to anactive catalyst.
 9. Method according to claim 8, wherein the solid corematerial is provided with at least one charged ionic species, preferablya charged polymeric ionic species, prior to applying the shell of zincoxide.
 10. Method according to claim 9, wherein an oxidic material otherthan zinc oxide, preferably of silica, is applied to the layer of thecharged ionic species, prior to applying the shell of zinc oxide. 11.Method according to claim 8, wherein a further layer of ionic species isapplied over the oxidic material, prior to applying the shell of zincoxide.
 12. Method according to claim 9, wherein a cationic material isfirst applied to the oxidic core material, followed by application of ananionic material or vice versa.
 13. Method according to claim 12,wherein the cationic material is poly diallyldimethyl ammoniumchloride.14. Method according to claim 12, wherein the anionic material is apolystyrene sulfonate salt.
 15. Catalyst obtained by a method accordingto claim
 8. 16. Process for producing liquid hydrocarbons using aFischer Tropsch process in the presence of a Fischer Tropsch catalystbased an active catalytic active material of one or more of the metalscobalt iron, ruthenium and/or nickel, the improvement comprising using acatalyst according to claim
 1. 17. Process according to claim 16,wherein the process is done in a slurry reactor, a loop reactor, abubble column or a fluid bed reactor.